All cultivated forms of lettuce belong to the highly polymorphic species, Lactuca sativa, which is grown for its edible head and leaves. As a crop, lettuces are grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, scorzonera, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of pests and diseases such as downy mildew (Bremia lactucae). Every year this disease leads to millions of dollars of lost lettuce crop throughout the world. Downy mildew (Bremia lactucae) is highly destructive on lettuce grown at relatively low temperature and high humidity. Downy mildew is caused by a fungus, Bremia lactucae, which can be one of the following strains: NL1, NL2, NL4, NL5, NL6, NL7, NL10, NL12, NL13, NL14, NL15, NL16, B1:17, B1:18, B1:20, B1:21, B1:22, B1:23, B1:24, B1:25, B1:26, B1:27, B1:28, B1:29, B1:30, B1:31, B1:32 (Van Ettekoven, K. et al., “Identification and denomination of ‘new races’ of Bremia lactucae,” In: Lebeda, A. and Kristkova, E (eds.), Eucarpia Leafy Vegetables, 1999, Palacky University, Olomouc, Czech Republic, pp. 171-175; Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants); IBEB press release “New race of Bremia lactucae B1:27 identified and nominated”, May 2010; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), “New race of Bremia lactucae B1:28 identified and nominated”, March 2011; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), IBEB press release, “New races of Bremia lactucae, B1:29, B1:30 and B1:31 identified and nominated”, August 2013; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), IBEB press release, “A new race of Bremia lactucae, B1:32 identified and nominated in Europe”, May 2015), Ca-I, Ca-IIA, Ca-IIB, Ca-III, Ca-IV (Schettini, T. M., Legg, E. J., Michelmore, R. W., 1991. Insensitivity to metalaxyl in California populations of Bremia lactucae and resistance of California lettuce cultivars to downy mildew. Phytopathology 81(1). p. 64-70), and Ca-V, Ca-VI, Ca-VII, Ca-VIII (Michelmore R. & Ochoa. O. “Breeding Crisphead Lettuce.” In: California Lettuce Research Board, Annual Report 2005-2006, 2006, Salinas, Calif., pp. 55-68).
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the fungus progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Of the various species of aphids that feed on lettuce, the currant-lettuce aphid (Nasonovia ribisnigri) is the most destructive species because it feeds both on the leaves of the lettuce as well as the heart of the lettuce, making it difficult to control with conventional insecticides. The lettuce aphid feeds by sucking sap from the lettuce leaves. Although direct damage to the lettuce may be limited, its infestation has serious consequences because the presence of aphids makes lettuce unacceptable to consumers. Although several known lettuce cultivars exhibit resistance against downy mildew and currant-lettuce aphid, irrespective of lettuce type, all the lettuce cultivars affected produce a limited number of leaves that are generally of unequal size and diminished quality with respect to color and shape. This is a distinct disadvantage for processing purposes because leaves either need to be sorted based on size, or they need to be cut into a smaller, more uniform size. The first option requires additional labor, with not all sizes being usable. The second option has the disadvantage that it creates many cut surfaces, which then are subject to wound-induced browning, thus resulting in a greatly reduced shelf-life.
Growers of lettuce varieties that are intended for the processing industry are always looking for varieties which combine characteristics such as resistances and leaf color, with a higher yield. Varieties that produce more harvestable biomass per acre are thus desired.
The production of packaged salad mixes, for example, involves harvesting and processing of lettuce, which induces a strong wound response on the cut surfaces. Such a wound-induced response can lead to a rapid deterioration of the processed product. This deterioration is manifested as a brown or pink discoloration at, or adjacent to the wound surface. The brown or pink color that develops over time on the cut leaf surface makes the product highly unattractive to potential customers. The wound-induced surface discoloration can in part be countered by reducing oxygen levels in the packaging.
However, using specialized packaging is costly and may result in anaerobic conditions. Anaerobic conditions may reduce shelf life and may promote the growth of micro-organisms that produce a bad smell, reducing the overall attractiveness and taste of the product. Therefore lettuce plants that exhibit reduced wound-induced discoloration of cut leaf surfaces is desirable. Such lettuce plants stay visually more attractive post-processing, and the need for reduced oxygen levels in the packaging of the cut leaves of these lettuce plants is minimized.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.